Conventionally, spark plugs are used in internal combustion engines for ignition. A general spark plug is such that an insulator which holds a center electrode within an axial hole thereof is held by a metal shell so as to surround circumferentially a circumference thereof, so that a spark discharge gap is formed between a ground electrode which is joined to the metal shell and a center electrode. In addition, a spark discharge generated in the spark discharge gap ignites an air-fuel mixture.
In recent years, there are demands on reduction in size and diameter of spark plugs to secure the degree of freedom in designing automotive engines for increase in output and fuel economy thereof, and reductions in diameter and thickness of metal shells and insulators have been attempted. As one of measures taken to achieve the attempt, although a measure is considered in which respective constituent components of a conventional spark plug are made smaller in size with their shapes remaining as they are, there is a possibility that a reduction in strength of the individual constituent components is called for in the event that the constituent components are simply reduced in size. Because of this, the sizes of the individual constituent components are balanced against each other so as to secure the strengths of the constituent components within the limited dimensions of the spark plug.
When the thickness of the insulator is reduced in association with the reduction in diameter of the spark plug, resulting in a reduction strength (rigidity) of the insulator, in the event that an external force is applied to the insulator in a direction perpendicular to an axial direction (a bending direction), cracks or fractures tend to be generated easily. There is a possibility that the external force in the bending direction is applied to the spark plug in the event that a mounting tool is brought into collision with a rear end side body portion (an insulator head portion) of the insulator which is exposed from a rear end of the metal shell when the spark plug is mounted in the engine. In addition, although a terminal metal base, which is connected electrically with a center electrode, is exposed at a rear end of the insulator, a plug cap attached to a lead wire for applying a voltage to a spark discharge gap is fitted on the terminal metal base after the spark plug has been mounted in the engine. When the spark plug is subjected to vibrations generated in association with the driving of the engine in this state, a load due to the weight of the plug cap is generated, leading to a possibility that the external force in the bending direction is applied to the rear end side body portion of the insulator.
To make it difficult for cracks or fractures to be generated in the insulator even though the external force in the bending direction is applied to the rear end side body portion of the insulator in the way described above, it may be good to secure the thickness of a portion of the rear end side body portion where the outside diameter of the rear end side body portion becomes minimum (a minimum outside diameter portion) by regulating the outside diameter and an inside diameter of the axial hole (a central through hole) at the outside diameter minimum portion. Further, it is desired to secure the strength against the external force applied to the insulator in the bending direction by regulating a modulus of section of the portion of the rear end side body portion where the outside diameter becomes minimum (for example, refer to Patent Document 1.).    Patent Document 1: JP-A-2006-100250